The silicon bipolar transistor has been the device of choice for high power applications in motor drive circuits, appliance controls, robotics and lighting ballasts. This is because bipolar transistors can be designed to handle relatively large current densities in the range of 200 to 50 A/cm.sup.2 and support relatively high blocking voltages in the range of 500-2500V.
Despite the attractive power ratings achieved by bipolar transistors, there exist several fundamental drawbacks to their suitability for all high power applications. Bipolar transistors are current controlled devices which require relatively large base control currents, typically one fifth to one tenth of the collector current, to maintain the transistor in an on-state mode. Proportionally larger base currents can be expected for applications which also require high speed turn-off. Because of the large base current demands, the base drive circuitry for controlling turn-on and turn-off is relatively complex and expensive. Bipolar transistors are also vulnerable to premature breakdown if a high current and high voltage are simultaneously applied to the device, as commonly required in inductive power circuit applications. Furthermore, it is relatively difficult to operate bipolar transistors in parallel because current diversion to a single transistor typically occurs at high temperatures, making emitter ballasting schemes necessary. This current diversion generally results from the decrease in on-state voltage drop across the bipolar device with further increases in operating temperature.
The silicon power MOSFET was developed to address this base drive problem. In a power MOSFET, the gate electrode provides turn-on and turn-off control upon the application of an appropriate gate bias. For example, turn-on in an n-type enhancement MOSFET occurs when a conductive n-type inversion layer is formed in the p-type channel region in response to the application of a positive gate bias. The inversion layer electrically connects the n-type source and drain regions and allows for majority carrier conduction between source and drain.
The power MOSFET's gate electrode is separated from the conducting channel region by an intervening insulating layer, typically silicon dioxide. Because the gate is insulated from the channel region, little gate current is required to maintain the MOSFET in a conductive state or to switch the MOSFET from an on-state to an off-state or vice-versa. The gate current is kept small during switching because the gate forms a capacitor with the MOSFET's channel region. Thus, only charging and discharging current ("displacement current") is required during switching. Because of the high input impedance associated with the insulated-gate electrode, minimal current demands are placed on the gate and the gate drive circuitry can be easily implemented.
Moreover, because current conduction in the MOSFET occurs through majority carrier transport only, the delay associated with the recombination of excess minority carriers is not present. Accordingly, the switching speed of power MOSFETs can be made orders of magnitude faster than that of bipolar transistors. Unlike bipolar transistors, power MOSFETs can be designed to simultaneously withstand high current densities and the application of high voltages for relatively long durations, without encountering the destructive failure mechanism known as "second breakdown". Power MOSFETs can also easily be paralleled, because the forward voltage drop of power MOSFETs increases with increasing temperature, thereby promoting an even current distribution in parallel connected devices.
The above-described beneficial characteristics of power MOSFETs are typically offset, however, by the relatively high on-resistance of the MOSFET's drift region for high voltage devices, which arises from the absence of minority carrier injection. As a result, a MOSFET's operating forward current density is typically limited to relatively low values, typically in the range of 40-50 A/cm.sup.2, for a 600 V device, as compared to 100-120 A/cm.sup.2 for the bipolar transistor for an identical on-state voltage drop.
On the basis of these features of power bipolar transistors and MOSFET devices, devices embodying a combination of bipolar current conduction with MOS-controlled current flow were developed and found to provide significant advantages over single technologies such as bipolar or MOSFET alone. One example of a device which combines bipolar and MOS characteristics is the Insulated Gate Bipolar Transistor (IGBT).
The IGBT combines the high impedance gate of the power MOSFET with the small on-state conduction losses of the power bipolar transistor. Because of these features, the IGBT has been used extensively in inductive switching circuits, such as those required for motor control applications. These applications require devices having wide forward-biased safe-operating-area (FBSOA) and wide reverse-biased safe-operating-area (RBSOA).
One disadvantage of an IGBT is its limited gate control on-state current density. This arises from the presence of a parasitic thyristor in its structure. At sufficiently high on-state current densities, this thyristor latches up, thereby losing gate control over the on current. This characteristic of IGBT's also limits the IGBT's surge current capability. Many proposals have been made for mechanisms to suppress the effectiveness of this parasitic thyristor at the cost of on-state voltage drop and/or switching speed.
Recent efforts have also included investigation of the use of silicon carbide (SiC) devices for power devices. Such devices include power MOSFETs such as are described in U.S. Pat. No. 5,506,421. Similarly, silicon carbide Junction Field Effect Transistors (JFETs) and Metal-Semiconductor Field Effect Transistors (MESFETs) have also been proposed for high power applications. See U.S. Pat. Nos. 5,264,713 and 5,270,554. These devices, however, have a forward voltage drop of approximately 3 volts as a minimum voltage drop. Thus, these devices are not suitable for all applications.
Silicon carbide IGBTs may further provide improved performance over other power devices because the forward voltage drop of the device does not increase with breakdown voltage at the same rate for an IGBT as for a MOSFET or JFET. As is illustrated in FIG. 1, the curve of breakdown voltage (BV) versus forward voltage drop (Vf) for a MOSFET/JFET 8 crosses the curve for a silicon carbide IGBT 9 at about 2000V. Thus, for breakdown voltages of greater than 2000V silicon carbide IGBTs may provide better performance in terms of forward voltage drop for the same breakdown voltage than silicon MOSFETs or JFETs.
While the characteristics of the silicon carbide IGBT indicate promise as a power device, such devices are currently limited in their applicability in silicon carbide. These limitations are a result of the present difficulties in fabricating good quality highly doped p-type silicon carbide substrates. Another limitation is the very low hole mobility in silicon carbide, thereby making it very susceptible to parasitic thyristor latch-up. Therefore, silicon carbide IGBTs are expected to have a low value of gate controlled on-state current density. Because the IGBT is typically a vertical device, the substrate on which the device is fabricated may be critical to device performance. The quality of the substrate material may be a limiting factor in the fabrication of quality devices. Thus, the difficulty in manufacturing good quality highly doped p-type silicon carbide substrates may presently limit the fabrication of IGBTs to n-type substrates.
In conventional power circuits it is desirable to have a device which may have the control voltage applied to the device to turn the device on and off referenced to ground rather than to a high positive voltage level. However, to provide an IGBT where the gate is referenced to the emitter of the device generally requires a highly doped p-type substrate. As is noted above, highly doped p-type substrates currently are more difficult to fabricate than n-type substrates in silicon carbide. With an n-type substrate a silicon carbide IGBT would have its gate voltage referenced to the collector voltage which, in a typical power circuit would be to a line voltage. Thus, present silicon carbide IGBTs may require more complex gate drive circuitry with level shifting components and result in more complex power circuits as a result of the structure of IGBTs, the electrical characteristics of silicon carbide and the limitations in fabrication of highly doped p-type silicon carbide substrates.
In light of the above discussion, there exists a need for improvements in high power silicon carbide devices.